The present invention relates to a method of fabricating a segmented cathode with striped electron emitting and nonemitting areas.
One way in which to increase amplification in a triode electron tube is to increase the number or density of grid wires. This method, however, has the consequence of undesirable increase in grid current. Thus, tetrodes and pentodes have been developed in order to increase the amplification while preventing the grid current from increasing, but such multi-electrode tubes are necessarily more complex in structure and hence more expensive to produce.
U.S. Pat. No. 3,814,972 issued June 4, 1974 to W. H. Sain and assigned to the assignee of the present invention illustrates an improved triode electron tube of relatively simple construction having a cylindrical cathode with its outer surface made of a plurality of alternatively located electron emitting and nonemitting areas which are electrically connected and remain equipotential with each other. Disposed coaxially around this cathode is a control grid composed of a cylindrical array of bar-like members with cross sectional dimensions and spacing within designated ranges and radially aligned with the nonemitting areas of the cathode so as to prevent the increase in grid current.
The conventional method of fabricating a cathode of this type is firstly to prepare a metal cylinder with shallow longitudinal grooves cut into the outer surface by means of hobbing or sawing, and equally spaced around the periphery, secondly to coat the entire cylinder with an electron emissive substance and finally to clean it so that the coating will stay in the grooves only. This method of "covering everything and wiping off the excess" may be adequate if the emissive coating substance is to completely fill the grooves so that the emissive and nonemissive surfaces in the final product are flush with each other. It is far from satisfactory, however, where the emissive surface must be recessed with respect to the adjacent nonemissive surfaces so that better focusing of the electron beam will be provided with the step acting as a lens to deflect electrons inwardly. Inadequacy of this method is more readily understandable where the height of the step must be accurately controlled.